The invention relates to a centrifugal pump, and a method of pumping medium consistency suspension in an effective manner. The invention is particularly applicable to the pumping of medium consistency finely comminuted cellulosic fibrous material suspensions (paper pulp), which should have a consistency of between about 6-20%.
Known pumps suitable for pumping medium consistency pulp are shown in U.S. Pat. Nos. 4,435,193 and 4,476,886, and in Canadian Patent 1,128,368. In use, these pumps are typically installed on the bottom of high density towers, are installed on the bottom or side or horizontal position in long chutes (e.g., 10 meters or greater), or are installed in short chutes. When the pumps are associated with high density towers or long chutes, sufficient head is provided so that the simplest form of pump (such as in Canadian Patent 1,128,368), which has no vacuum system, may be utilized. Such pumps operate very successfully. However, for pumps associated with short chutes, and for some other situations, the head is not sufficient so that a vacuum system is required, such as shown in U.S. Pat. No. 4,435,193.
While the medium consistency pulp pumps with vacuum systems, such as shown in U.S. Pat. No. 4,435,198, can operate successfully, operation is not simple either from an operating point of view or from a maintenance point of view. The vacuum control system has to be properly maintained in order to ensure successful operation. In practice what typically happens is that when the operator has difficulty in pumping higher consistency pulp, he adds dilution liquid and if the pump then works properly he leaves it running at the lower consistency. While the system then works, since it is working at a lower consistency than desired the advantages of pumping at the higher consistency are not achieved.
According to the present invention, a pump, and method of pumping, are provided which provide a significant boost in head so that basically the same pump as disclosed in Canadian Patent 1,128,368 and U.S. Pat. No. 4,476,886 can be utilized in short chutes, and other situations where a vacuum system is typically necessary, without need of a vacuum system.
According to the present invention, there is provided a method of pumping a suspension having a consistency of about 6-20%. The method comprises the steps of continuously acting upon the suspension to progressively: (a) Effect initial fluidization of the suspension. (b) Effect removal of extreme pre-rotation of the fluidized suspension and define a definite directional flow angle of the suspension. (c) Effect a first fluidizing pumping of the suspension moving in a definite directional flow, in a first stage, to pump the suspension directly to a second stage; and (d) effect a second fluidizing, centrifugal, pumping of the suspension at the second stage.
The pump according to the present invention, which allows the practice of the method of the invention, includes a housing having an inlet and an outlet, and a shaft mounted for rotation within the housing about an axis of rotation. A first set of impeller vanes are mounted to the shaft within the housing adjacent the housing inlet, and a second set of impeller vanes are mounted to the shaft within the housing between the first set and the outlet. The second set of vanes may be essentially identical to the construction of the impeller of a conventional pump for medium consistency pulp. A third set of vanes are also provided stationarily mounted to the housing between the inlet and the first set of vanes, and a fourth set of vanes are mounted to the shaft and extend out of the housing. The fourth set of vanes typically would contact pulp in a vessel above the pump housing to draw the pulp into the pump housing, and effect pre-fluidization of the pulp. The stationary vanes remove extreme pre-rotation of the suspension and define a definite directional flow angle of the suspension to the first set of impeller vanes. The first set of impeller vanes axially direct the pulp into contact with the second set of impeller vanes, which effect pumping of the pulp out of the pump housing outlet.
The operation of the pump according to the present invention is significantly improved compared to prior medium consistency pulp pumps. In the prior pumps, the pre-fluidizing vanes protrude into the chute about three inches, and have a 20.degree.-30.degree. helical angle to help draw the pulp into the pump inlet. However, part of the incoming stock, before it reaches the inlet, is thrown outwardly by a powerful centrifugal force. This incoming, and then laterally rejected stock, creates a continuous circulation, having a doughnut shape, around the periphery of the inlet, interfering with the incoming stock and thereby restricting the area of the free inlet. This causes, for a given GPN, higher velocity of the incoming pulp which in turn lowers the available NPSH and inlet pressure.
The stationary guide vanes in the pump according to the present invention, by removing the pre-rotation ahead of the impeller and changing the turbulent and chaotic condition of the stock into a directional flow into the impeller at a definite angle of entrance, provide greatly improved suction conditions and enhanced efficiency. The first set of impeller vanes can impart around a 40-foot additional head, for example, at 1,200 GPN and 2,000 RPM, and will require an additional 40-horsepower. This typically would be sufficient to allow elimination of the vacuum system in many situations.
The pump according to the present invention may easily be applied to situations where prior art medium consistency pumps have been utilized. All that is necessary to provide a pump according to the present invention, as opposed to the prior art pump, is to utilize a housing extension which attaches to the top part of the housing of the conventional pump, provide a longer shaft, and attach the first set of impeller blades to the shaft above the second set of impeller blades, and attach the stationary vanes to the housing upper part. The pre-fluidizing vanes may then be bolted, or otherwise connected, to the end of the shaft.
It is the primary object of the present invention to provide for the effective pumping of medium consistency pulp, and like suspensions. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.